1. Field of the Invention
The invention concerns a blade mounting extension for a reciprocating saw such as a portable handheld electric saw. The blade mounting attaches at one end to a conventional flat saw blade and attaches at an opposite end to the conventional blade mounting fixture that otherwise would receive the saw blade. The inventive extension places the centerline of the saw blade at a lateral offset from the centerline of the blade mounting fixture, as well as spacing the blade linearly from the mounting fixture.
2. Prior Art
A range of portable power tools provide for reciprocating motion and typically are powered electrically or pneumatically. The driving portion of such a power tool has a housing that requires a considerable lateral width and height, as well as depth in the direction of reciprocation, to support, guide and drive the shaft that carries the reciprocating element. An electric reciprocating tool, for example, typically has a rotating-armature electric motor and an eccentric crank or cam arrangement for converting rotation of the motor into reciprocation of a fixture to which one end of the working tool element, such as a serrated saw blade, is attached.
There are a number of popular power tools, variously known as scroll saws, saber saws, jig saws, hole saws, etc., that reciprocate elongated blades. Some such saws have blades that are placed near a front end of a housing that is mounted above a support plate. The housing is elongated in one direction and the blade extends perpendicular to the elongation of the housing. Another popular form has a housing that is elongated in the same direction as the longitudinal extension of the blade. Both types of saws usually have a gripping handle on the housing on a side opposite from the blade, the handle being parallel to the normal cutting direction. In some arrangements, the blade can be oriented selectively to face its cutting edge one direction or another relative to the housing, e.g., forward or backward or laterally.
In addition to the drive shaft for attachment of the blade or other working element, the housing carries the power generation element (e.g., electric motor), the mechanical supporting bushings for the reciprocating drive shaft, and any associated linkage and gearing parts. These provisions space the drive shaft from some or all of the outer surfaces of the housing of the tool. The drive shaft is typically centered between lateral outside surfaces of the housing. As a result, the longitudinal center line of the blade, corresponding to the line that is cut by the saw, is often located along a center line of the housing. Even if offset, the drive shaft axis generally is not placed at or near the lateral outside edge of the tool. As a result, the housing of the tool can interfere with positioning of the blade in connection with some desirable cutting situations.
Interference due to the saw housing is apparent in the example of making a flush cut on a surface. Assuming that a structure protruding from a surface is to be cut off flush with the surface, the saw blade needs to be oriented parallel with the surface and placed directly against the surface, so as to make a cut immediately along the surface at the proximal part of the protruding structure. If a blade that is centered between lateral sides of a housing having a given width, should be brought parallel to a surface, the closest that the blade can come to the surface is a distance equal to the spacing between the blade and the lateral outside of the housing. If the blade is centered, this distance is half the width of the housing, e.g., several cm.
As a result of potential interference due to the width of the housing, the operator may attempt to incline the tool housing and lay the blade laterally against the surface while causing the blade to bend. This is wearing on the blade, which advantageously should be disposed in a straight line, to bear the alternating tension and compression forces caused by the reciprocating sawing motion, rather than being caused to flex. Bending the blade to cut also is hard on the tool, applying off center forces to the drive shaft, to the mounting of the blade to the drive shaft as well as the mounting of the drive shaft in the tool.
Attempting a flush cut by inclining the tool also often does not result in a clean flush cut. The result may be an inclined cut or stub rather than a neat flush cut. The surface adjacent to the protrusion may be marred. What is needed is a way to align the operating position or cutting line of the blade with the lateral outer edge of the housing that carries the driving mechanism for the blade, even in a tool that has a cutting line spaced well inwardly from the lateral side of the housing, while driving the blade in a straight reciprocating motion.
In a saw of the type having a blade that is elongated perpendicular to the elongation of the housing, the blade driving shaft can be placed near the front or nose of the tool, as typified by U.S. Pat. No. 4,566,190—Isakson. This placement is helpful to enable cutting close to an abutment, at least if the blade can be re-oriented parallel to the abutment. The driving shaft is still spaced inwardly from the closest edge (the nose) at least by the thickness of the bushing carrying the driving shaft. In order to align the blade cutting line with the outside of the housing at the nose, a support block is provided at the end of the driving shaft, for a screw or clamp fixture positioning the blade at a space from the remaining surface of the driving shaft. Isakson has a solid support block mounted on the end of the reciprocating driving shaft, the blade being fastened on an outer face of the block and thus spacing the cutting line of the blade from a cylindrical projection of the surface of the driving shaft. Another sort of solid block is disclosed in U.S. Pat. No. 4,553,306—Mineck. In U.S. Pat. No. 3,028,890—Atkinson et al., the blade is attached to the drive shaft on the outside of a square tube. The square tube provides at least two optional orientations for the blade, both spaced from the projection of the driving shaft. Atkinson also teaches a laterally asymmetrical housing so that from the outset the driving shaft is placed closer to one of the laterally opposite sides of the housing than to the other.
Mounting the blade on a cutting line that is laterally spaced from the reciprocating driving shaft has some of the same problems as attempting to tilt the saw at an incline in order to make a flush cut. The reciprocating forces associated with moving the blade in a sawing motion are transmitted in part as lateral and bending forces on the drive shaft and its mounting. The resultant force includes torque applied to the end of the driving shaft around a torque axis perpendicular to the longitudinal axis of the driving shaft. The severity of this torque is proportional to the lateral spacing of the blade center line from the drive shaft axis, this lateral spacing equating with the length of a lever arm. As the saw reciprocates, torque is applied in alternating directions.
U.S. Pat. No. 5,809,657—Mortensen teaches a blade adaptor that displaces the cutting line of a blade forward from the nose of a saw of the type having a blade aligned perpendicular to and at the front or nose of the housing. The blade is parallel to the drive shaft but is spaced by a distance of at least several cm. forward from the drive shaft. The adaptor forms more or less of a right angle or L-shape. The offset distance (the foot of the L-shape) defines a moment arm. Reciprocation forces applied to the blade by cutting are translated into a rocking or bending action at the connection to the drive shaft with a lever or multiplying factor proportional to the offset distance. These forces wear and loosen the clamping mechanism, screws or other mechanical device used for attachment at the drive shaft. The forces also wear and loosen the mounting of the drive shaft and the mechanism by which the tool drives the blade. These forces produce friction and metal fatigue and generally reduce the useful life of the tool.
In U.S. Pat. No. 3,260,290—Happe et al., a blade adaptor attachment holds the blade laterally alongside and outside of the width of the tool housing. The offset in this case is by an even greater distance than the nose-forward offset in Mortensen. Happe addresses the leverage of the reciprocation force by providing a guide shaft in addition to the drive shaft. The driving shaft and guide shaft are parallel and spaced. This solution reduces flexing at the attachment of the adaptor to the reciprocating drive shaft, but it requires a special non-standard tool effectively having two parallel shafts to guide the blade. The use of two shafts increases operational friction. The leveraged torque from the offset remains a factor, even if better borne by two shafts than by one as in Mortensen. The torque tends to produce alternating racking of the two parallel shafts, instead of flexing specifically at the attachment of the adaptor to the drive shaft.
It would be advantageous to provide a flush cutting blade rig that can be used on any reciprocating blade tool. Many popular reciprocating blade tools have a substantial distance between the drive shaft and the lateral edge of the housing and are not generally designed for flush cutting. It would be advantageous to adapt them for flush cutting, but not to damage the tool as a result. A laterally offset fixture is needed that positions the blade at a sufficient distance from the drive shaft to enable flush cutting with most types of tools, yet attaches to the tool simply, preferably with no more complication than attaching the blade to the driving shaft. Having made such an attachment, provisions need to be made somehow to minimize or reduce the adverse effects of the flexing and racking forces that could be produced by torque leveraged according to the distance at which the blade is offset from the driving shaft.